The present invention relates generally to fluid pressure sensing chambers and more specifically to fluid pressure sensing chambers used in ophthalmic surgical equipment.
When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. During this procedure, a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquifies or emulsifies the lens so that the lens may be aspirated out of the eye. The diseased lens, once removed, is replaced by an artificial lens.
A typical ultrasonic surgical device suitable for ophthalmic procedures consists of an ultrasonically driven handpiece, an attached cutting tip, and irrigating sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubings. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubings supply irrigation fluid to and draws aspiration fluid from the eye through the handpiece assembly.
The operative part of the handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece by flexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. The nosecone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve.
In use, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined width in the cornea, sclera, or other location. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source, usually a peristaltic pump, in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the cutting tip and horn bores and the aspiration line and into a collection device. The aspiration of emulsified tissue is aided by a saline flushing solution or irrigant that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the cutting tip.
Prior art devices have used sensors that detect irrigation pressure or aspiration vacuum. Based on the information from these sensors, the surgical console can be programmed to respond in order to make the surgical procedure more efficient and safer. In order to reduce the risk of contamination by the aspirated fluid, recent surgical systems use closed pressure sensors, in which the fluid does not come into contact with the load cell or other device used to sense the fluid pressure. One such pressure sensor is illustrated in U.S. Pat. No. 5,392,653 (Zanger, et al.). Overall performance of such closed pressure sensors, however; depends in large part on purging all of the air from the system. Air is much more compressible than the irrigating solution used in surgery, and air pockets or bubbles add compliance to the system. Compliance results in undesirable pressure variations and fluctuations. Common methods of assisting in purging air from sealed liquid systems (or “priming” the system) include avoiding sharp edges and abrupt shape changes within the system as well as filling the system with liquid from the bottom or low point of the system. This allows the air to escape out of the top of the system as the systems fills with liquid from below because, as one skilled in the art will recognize, saline solutions are heavier than air. The inventors of the present invention have discovered that the initial priming of a pressure sensor chambers found within closed surgical fluidic systems is relatively easy, but if bubbles of air are allowed to enter the chamber (for example, if the surgical handpiece is changed mid-procedure), these air bubbles are extremely difficult to purge from the system. This difficulty is the result of the surface tension of the air bubble (as opposed to the unencapsulated air generally involved in the initial priming of the system) causing the bubble to be relatively robust and not easily broken and drawn out of the pressure sensing chamber once introduced. In addition, the surface tension of the liquid “film” surrounding the air bubble causes the bubble to have a “tackiness”, causing the bubble to want to stick or adhere to surfaces within the system and resist further movement, even with very high flow rates. One reference, U.S. Pat. No. 6,059,765 (Cole, et al.) has suggested that certain chamber shapes and outlet locations may assist in the removal of air from surgical systems. The inventors have found that the chamber shapes and designs discussed in this reference are insufficient to assure that air bubbles can be purged from the system because fluid flow through the chamber tends to follow defined pathways. Fluid contained within the chamber out side of these defined pathways tends to be relatively stagnant. Any air bubbles contained within these stagnant pools of fluid will adhere to the boundaries of the chamber and resist being purged from the chamber.
Accordingly, a need continues to exist for a pressure sensing chamber that helps purge air bubbles that may be introduced into the chamber.